The present invention relates to switching video signals, and more particularly to switching video of two different standards.
Some users of a video matrix find it desirable to be able to switch video of two different standards, i.e. the Phase Alternate Line standard (PAL), which is a 50 Hz/625 line standard, and the National Television System Committee standard (NTSC), which is a 60 Hz/525 line standard. For example, a video production house that works with international customers may be dealing with different video standards. Or, the user may wish to switch video of the same standards, but with different timings.
The problem arises in that communication from a local controller is referenced to a vertical interval strobe, say VI1, created from a single system reference, say Reference 1, by a vertical interval (VI) pulse generator. Control of the matrix by a local controller works in the following way. Reference 1 may be either the 50 Hz or 60 Hz standard, and is typically a color black signal. The pulse of VI1 typically occurs during the middle of line 10 for 60 Hz systems, or the middle of line 6 for 50 Hz systems. Communications are sent to a crosspoint switch while VI1 is not pulsing, and the final strobing, or setting of the switch takes place when the pulse occurs. If all inputs are timed to the system reference, all switches occur at the correct time. However, if the user wishes to have sources that are of a different reference than Reference 1, say Reference 2, there needs to be a way to ensure that the video switches during the correct line for sources referenced to Reference 2, even though communications still occur according to Reference 1.
There are currently two methods of dealing with this problem. The first, as shown in FIG. 1, is to simply switch signals of a second standard according to the first and allow them to glitch, but this is not acceptable. The second, as shown in FIG. 2, is to have two separate matrices, each with its own local controller, vertical interval pulse generator and reference. Often times, however, the user will have a large portion of his or her system switching one standard, while only a few sources are of the other standard. This makes it unattractive to have a separate matrix with its own local control.
What is desired is a system for switching video of two different standards that allows a single controller to still write crosspoint data at a rate according to a first reference, but allows switching to occur at the vertical interval of the first reference or a second reference of a same or different standard.